is this an emachines 420?..
ive had a couple of these to fix with faulty motherboards..
the problem is it might not just be your motherboard, in some cases its the power supply unit aswell and sometimes even other hardware (i.e HDD, CPU etc..)
i found out by testing the mother board with a different psu and vice versa but if you havent got a spare mobo/psu i suggest talking it to a local shop for them to have a look at it
Luke Ward
February 2008